Complementary metal-oxide-semiconductor (CMOS) image sensors are widely used in the image-related products such as cameras and video recorders. CMOS image sensors are able to capture photons, and generate electrical signals from the received photons. To ensure the sensitivity of the CMOS image sensors, the leakage currents of the CMOS image sensors need to be well controlled. One of the methods of controlling the leakage currents is to define electrical insulations between the image sensors. For example, deep p-well regions are formed to have a grid pattern to insulate the image sensors that are formed in the grids.
The deep p-well regions are formed by implanting a p-type impurity into deep regions of a semiconductor substrate. Accordingly, the energy for the implantation is high. This requires that the mask used for the implantation to be thick. Conventionally, a thick oxide layer is typically formed over the semiconductor substrate, and then etched to form trenches that have a grid pattern, wherein the p-type impurity is implanted to the underlying semiconductor substrate through the trenches in the thick oxide layer.
The deep p-well regions need to be narrow, so that the chip area occupied by the insulation structure is as small as possible, and the respective image sensor wafer has a high fill factor. This requires that the trenches in the thick oxide layer to be narrow, and that the sidewalls of the trenches to be vertical. This requirement can be satisfied when the critical dimension of the trenches is greater than about 0.28 μm. When the critical dimension of the trenches is reduced to small than about 0.18 μm, however, the profiles of the trenches were no longer satisfactory. This is partially due to the limitation of the wavelength of deep UV light that is used for the lithography process, which is not suitable for exposing thick photo resist having such a small critical dimension. In addition, when the critical dimension of the trenches is too small, the limitation also incurs in the etching of the thick oxide layer, wherein there may be some oxide residue left at the bottom of the trenches with small critical dimension. It is difficult to etch the thick oxide layer to form high aspect ratio trenches having vertical sidewalls.